Optical Coherence Imaging Systems Having a Reduced Effective Linewidth

ABSTRACT

Frequency domain optical coherence imaging systems have an optical source, an optical detector and an optical transmission path between the optical source and the optical detector. The optical transmission path between the optical source and the optical detector reduces an effective linewidth of the imaging system. The optical source may be a broadband source and the optical transmission path may include a periodic optical filter.

RELATED APPLICATIONS

The present application is a continuation of co-pending U.S. patentapplication Ser. No. 13/168,286 filed Jun. 24, 2011 (now U.S. Pat. No.______), which is a continuation of U.S. patent application Ser. No.12/551,782 filed Sep. 1, 2009 (now U.S. Pat. No. 7,990,541), which is acontinuation of U.S. patent application Ser. No. 11/495,226, filed Jul.28, 2006, (now U.S. Pat. No. 7,602,500) and which application alsoclaims the benefit of U.S. Provisional Application No. 60/703,376 filedJul. 28, 2005, the disclosures of which are hereby incorporated hereinin their entirety by reference.

BACKGROUND

The present invention relates to imaging systems and, more particularly,to optical-coherence imaging systems.

A variety of approaches to imaging using optical coherence tomography(OCT) are known. Such systems may be characterized as Fourier domain OCT(FD-OCT) and time domain OCT (TD-OCT). FD-OCT generally includes sweptsource (SS) and spectral domain (SD), where SD systems generally use abroadband source in conjunction with a spectrometer rather than a sweptlaser source and a photodiode(s). TD systems generally rely on movementof a mirror or reference source over time to control imaging depth byproviding coherence depth gating for the photons returning from thesample being imaged. Both systems use broadband optical sources,producing a low aggregate coherence that dictates the achievableresolution in the depth, or axial, direction.

These imaging techniques are derived from the general field of OpticalLow Coherence Reflectometry (OLCR). The time domain techniques arespecifically derived from Optical Coherence Domain Reflectometry. Sweptsource techniques are specifically derived from Optical Frequency DomainReflectometry. Spectral domain techniques have been referred to as“spectral radar.”

In contrast to time domain systems, in FD-OCT the imaging depth may bedetermined by Fourier transform relationships between the acquiredspectrum, rather than by the range of a physically scanned mirror,thereby allowing concurrent acquisition of photons from all imageddepths in the sample. Specifically, in FD-OCT, the optical frequencyinterval between sampled elements of the spectrum may be used to controlthe imaging depth, with a narrower sampling interval providing a deeperimaging capability.

In addition to total bandwidth, which generally controls the axialresolution, and sampling interval, which generally controls the imagingdepth, a third parameter, the effective sampled linewidth, generallycontrols a quality of the image as function of depth. As used herein,references to “linewidth” refer to the effective sampled linewidthunless indicated otherwise. As the effective sampled linewidth at eachsampled interval is increased, the effective sampled coherence lengthdecreases, which may produce a detrimental envelope of decreasingsignal-to-noise ratio across the imaged depth. This behavior is commonlyknown as fall-off and it is generally desirable to minimize this signalquality fall-off.

SUMMARY OF THE INVENTION

Embodiments of the present invention include frequency domain opticalcoherence imaging systems having an optical source, an optical detectorand an optical transmission path between the optical source and theoptical detector. The optical transmission path between the opticalsource and the optical detector reduces an effective linewidth of theimaging system. The optical source may be a broadband or low coherencesource and the optical transmission path may include a periodic opticalfilter.

In other embodiments, the optical detector is a plurality of opticaldetectors and the optical transmission path includes a spatial maskpositioned proximate the optical detectors. The plurality of opticaldetectors may be a spectrometer and the spatial mask may be positionedin the spectrometer. The spectrometer may be a tunable spectrometer.

In further embodiments, the optical source is a broadband source and theoptical transmission path includes an optical splitter having a firstside including a port coupled to a source arm and a port coupled to adetector arm and a second side having a port coupled to a reference armand a port coupled to a sample arm. The optical transmission pathfurther includes a periodic optical filter. The periodic optical filtermay be positioned in the detector arm between the optical splitter andthe optical detector. The optical detector may be a first spectrometerand a second spectrometer and the periodic optical filter may have afirst output coupled to the first spectrometer and a second outputcoupled to the second spectrometer. The periodic optical filter may bean interleaver having a finesse of two or of greater than two. Theperiodic optical filter may have a free spectral range thatsubstantially matches a pixel spacing of the first and secondspectrometer.

In other embodiments, the optical detector is a spectrometer and thesystem further includes an optical switching device in the detector armthat selectively couples a plurality of outputs of the periodic opticalfilter to the spectrometer. The periodic optical filter may be aninterleaver having a finesse of two and the plurality of outputs of theperiodic filter may be a first comb output and a second comb output,each having substantially equal peak widths and the second comb outputmay be offset from the first comb output by about half of a freespectral range of the periodic optical filter.

In yet further embodiments, the periodic optical filter is a cascadedplurality of optical filters and the optical detector is a firstspectrometer and a second spectrometer. The cascaded periodic opticalfilters have a first output coupled to the first spectrometer and asecond output coupled to the second spectrometer. The first output maybe a first set of outputs from one of the cascaded periodic opticalfilters and the second output may be a second set of outputs fromanother of the cascaded periodic optical filters. The system may furtherinclude a first optical switching device coupling the first set ofoutputs to the first spectrometer and a second optical switching devicecoupling the second set of outputs to the second spectrometer. Thecascaded periodic filters may be configured to generate the first andsecond set of outputs by splitting an optical signal input into aplurality of combs that are periodic and offset from each other. Thefirst and second spectrometer may have a pixel spacing substantiallyequal to a largest free spectral range of the cascaded periodic opticalfilters. The cascaded periodic optical filters may include at least oneoutput not coupled to the first or the second spectrometer and an imagefalloff of the system may be determined based on a filter width of anarrowest of the periodic optical filters and a falloff of the systemmay be determined based on a minimum optical interval between pixels ofthe optical detector.

In other embodiments, the periodic optical filter is a tunable periodicoptical filter having a selectable output wavelength comb spectra. Thetunable periodic optical filter may have a selectable free spectralrange and/or finesse. The tunable periodic optical filter may have atuning rate selected based on a desired image rate for the system. Thetunable periodic optical filter may have a number of steps in a scanselected to provide a desired resolution and/or falloff.

In further embodiments, the periodic optical filter is positioned in thesource arm between the optical splitter and the optical source.

In yet other embodiments, the system further includes a second periodicoptical filter. The first periodic optical filter is positioned in thereference arm or the sample arm and the second periodic optical filteris positioned in a different arm of the system. The periodic opticalfilters may be interleavers having a substantially same free spectralrange (FSR) and a finesse of at least two and the periodic opticalfilters may be offset by about one quarter of the FSR. One or both ofthe periodic optical filters may be a tunable periodic optical filter.

In other embodiments, the optical source is a plurality of tunableoptical sources and the optical detector is a plurality of opticaldetectors, ones of which are optically coupled to respective ones of thetunable optical sources by the optical transmission path. The opticaltransmission path may include an optical multiplexer coupling theplurality of tunable optical sources to a source arm of the system andan optical demultiplexer coupling the plurality of optical detectors toa detector arm of the system. A periodic optical filter may be providedin the source arm, the detector arm, a sample arm of the system and/or areference arm of the system.

In yet further embodiments, the optical source includes superluminescent diode sources and the optical detectors are spectrometers,ones of which are optically coupled to a selected wavelength rangeemitted by the super luminescent diode sources. The optical transmissionpath may include an optical multiplexer coupling the super luminescentdiode sources to a source arm of the system and an optical demultiplexercoupling the spectrometers to a detector arm of the system. A periodicoptical filter may be provided in the source arm, the detector arm, asample arm of the system and/or a reference arm of the system.

In other embodiments, the optical source includes a plurality of opticalsources and the optical detector includes a plurality of opticaldetectors. The optical transmission path includes an optical splitterhaving a first side including a port coupled to a source arm and asecond side having a port coupled to a reference arm and a port coupledto a sample arm and an optical multiplexer/demultiplexer coupling theplurality of optical sources and optical detectors to the source arm.The plurality of optical sources and optical detectors may be aplurality of optical source and detector pairs and the opticaltransmission path may further include a plurality of circulatorscoupling respective ones of the optical source and detector pairs to theoptical multiplexer/demultiplexer.

In further embodiments, the optical source is a broadband source and theoptical detector is a spectrometer. The optical transmission pathincludes an optical splitter having a first side including a portcoupled to the spectrometer and a second side having a port coupled to areference arm and a port coupled to a sample arm and a periodic opticalfilter arrangement coupling the sample arm and the reference arm to thebroadband source. The optical splitter may be a first optical splitterand the periodic optical filter arrangement may include a first periodicoptical filter coupled to the reference arm and a second periodicoptical filter coupled to the sample arm. A second optical splitter maybe provided having a first side including a port coupled to thebroadband source and a second side including a port coupled to the firstperiodic optical filter and a second port coupled to the second periodicoptical filter. The first periodic optical filter and the first opticalsplitter may be coupled to the reference arm by a first circulator andthe second periodic optical filter and the first optical splitter may becoupled to the sample arm by a second circulator.

In yet other embodiments, optical coherence imaging systems include anoptical splitter, an optical source and an optical detector. The opticalsplitter has a first side including a port coupled to a source arm and aport coupled to a detector arm and a second side having a port coupledto a reference arm and a port coupled to a sample arm. The opticalsource is coupled to the source arm and generates a comb output havingan associated spacing and linewidth. The optical detector is coupled tothe detector arm. The optical detector has a spacing and a bandwidthselected based on the associated spacing and linewidth of the opticalsource to reduce an effective linewidth of the imaging system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating an optical engine(system) according to some embodiments of the present invention.

FIG. 2 is a schematic block diagram illustrating an optical engine(system) according to other embodiments of the present invention.

FIG. 3 is a schematic block diagram illustrating an optical engine(system) according to other embodiments of the present invention.

FIG. 4 is a schematic block diagram illustrating an optical engine(system) according to further embodiments of the present invention.

FIG. 5A is a schematic block diagram illustrating an optical engine(system) according to other embodiments of the present invention.

FIG. 5B is a schematic block diagram illustrating an optical engine(system) according to further embodiments of the present invention.

FIG. 6 is a schematic block diagram of an optical engine (system)according to some embodiments of the present invention.

FIG. 7 is a schematic block diagram of an optical engine (system)according to further embodiments of the present invention.

FIG. 8 is a schematic block diagram of an optical engine (system)according to other embodiments of the present invention.

FIG. 9 is a schematic block diagram of an optical engine (system)according to further embodiments of the present invention.

FIG. 10 is a schematic block diagram of an optical engine (system)according to further embodiments of the present invention.

FIG. 11 is a schematic block diagram of an optical engine (system)according to further embodiments of the present invention.

FIG. 12 is a schematic block diagram of an optical engine (system)according to further embodiments of the present invention.

FIG. 13 is a schematic block diagram of an optical engine (system)according to further embodiments of the present invention.

FIG. 14 is a schematic block diagram of an optical engine (system)according to further embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Specific exemplary embodiments of the invention now will be describedwith reference to the accompanying drawings. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. The terminology used in the detailed description ofthe particular exemplary embodiments illustrated in the accompanyingdrawings is not intended to be limiting of the invention. In thedrawings, like numbers refer to like elements.

As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless expressly stated otherwise. Itwill be further understood that the terms “includes,” “comprises,”“including” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof. It will be understood thatwhen an element is referred to as being “connected” or “coupled” toanother element, it can be directly connected or coupled to the otherelement or intervening elements may be present. Furthermore, “connected”or “coupled” as used herein may include wirelessly connected or coupled.As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andthis specification and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

Some embodiments of the present invention provide optical systems(engines) for OCT that employ one or more optical filtering devices thatmay increase the image resolution, increase the image depth and/orreduce the image fall off. The filters may serve several functionsincluding: ensuring that each individual detector sees onelinewidth-managed source sample at any given moment; selecting whatportion of the spectrum is seen by a spectrometer, and; shaping thebandwidth seen by each pixel inside any of the spectrometers. Theadvantages of adding filtering in some embodiments may includeintrinsically superior image quality, improved performance with lessexpensive parts, improved fall-off by narrowing effective sampledlinewidth, higher resolution systems enabled by multiple sourcesspanning longer wavelengths and higher imaging speed based on thefurther advantages of parallel data acquisition, which may enableshorter scanning ranges for swept sources or multiple spectrometers eachacquiring data.

In some embodiments of the present invention an optical filter is addedto an otherwise conventional OCT engine. Optical filters are devicesthat selectively pass light based on the physical properties of thelight including the wavelength, wavemode, and/or polarization. Examplesof optical filters include thin film filters, fiber Bragg gratings,dichroic filters, arrayed waveguide filters, Fabry Perot filters,echelle gratings, interleavers, bulk diffraction gratings and others.These devices are commercially available from numerous companiesincluding large optical vendors, such as JDSU and Bookham.

Optical filters can be grouped into several categories including: Type1, filters having one input and two outputs that separate out one bandof wavelengths; Type 2, filters having one input and multiple outputsthat separate out bands of wavelengths, and; Type 3, filters having oneinput and two outputs that separate wavelengths in a periodic fashion.Type 1 includes thin film filters, dichroic filters, and fiber Bragggratings (with circulators). These filters are generally useful foradding or removing a particular wavelength from a group of wavelengths.By cascading these filters, multiple wavelengths may be combined orsplit apart although the insertion loss can limit the performance of thecascade.

Type 2 filters include arrayed waveguides (AWGs), echelle gratings, andbulk diffraction gratings. These filters generally split the input lightinto many wavelength groups with each group having its own output port.These filters are generally useful for combining or splitting manywavelengths with port counts up to 80 now available, and port counts upto 1080 have been reported. Spectrometers use this same principle with abulk diffraction grating spreading the spectrum across an array ofphotodiodes instead of an array of output ports.

Type 3 filters are periodic filters that pass evenly spaced wavelengthsout of one port and all other light out of the other port. Examplesinclude Fabry Perot filters and interleavers. It is possible to buildthese devices with a wide range of wavelength spacing and finesses.Interleavers available in the telecommunications industry typically havea finesse of 2 and a periodicity ranging from 200 GHz down to 25 GHz inthe 1550nm telecommunications window (0.2 nm to 1.6 nm)

Also included in some embodiments of the present invention are opticalswitching devices (or optical switches). Numerous technologyimplementations for optical switches can be used, including, but notlimited too, mechanical switches based on moving mirrors or prisms,micro-electro-mechanical systems (MEMS) based mirror switches, movingfiber switches, frustrated total internal reflection (FTIR) switches,switches based on phase changing, such as lithium niobate modulators,and switches based on thermal effects. Relevant parameters of theswitches may include the insertion loss of the switch and the switchingtime.

Some embodiments of the present invention will now be described withreference to the schematic block diagrams of FIGS. 1-14. Referring firstto the embodiments of FIG. 1, a periodic optical filter (POF) 108 isprovided in the detector arm (path) 103 to split the spectrum into twocombs having their associated spacing and width controlled by thecharacteristics of the POF 108. If a typical interleaver with a finesseof 2 is used as the POF 108, then there are two output combs that havesubstantially equal peak widths and are offset by one half of the freespectral range of the POF 108. One of the combs of wavelengths passes tothe spectrometer 109 and the offset comb of wavelengths pass to thespectrometer 110. Other POFs may be used that pass less that one halfthe total bandwidth to one output of the POF and more that half thetotal bandwidth to the other output of the POF.

Also shown in the embodiments of FIG. 1 is a low coherent source 100that provides the optical source to the source arm 102 to a polarizationbeam splitter (coupler) 101. However, it will be understood that, insome embodiments, a comb source may be used as the source 100 and theperiodic optical filter 108 may be omitted. The splitter 101 providesthe optical source light down a reference arm 104 to reference 106, suchas a mirror, and the sample arm 105 to the sample 107, illustratedschematically as a human eye in FIG. 1. The detector arm 103 couples theperiodic optical filter 108 to the splitter 101.

In some embodiments, the spectrometers 109 and 110 may be identical, butthey need not be. In some embodiments, the pixels inside thespectrometers are only illuminated by half of the wavelengths that theywould see without the POF. In other words, the free spectral range (FSR)of the POF 108 matches or nearly matches the pixel spacing, e.g.sampling interval, in the spectrometers 109 and 110.

The most common design of spectrometers generally results in pixels thatare evenly spaced in wavelength, whereas typical POFs are evenly spacedin frequency. This may be advantageous given that the data out of thespectrometer is typically resampled from wavelength spacing to frequencyspacing prior to further processing by the OCT engine. However, it willbe understood that, in some embodiments, the POF 108 is evenly spaced inwavelength and the spectrometers 109 and 110 are evenly spaced infrequency, for example, by chirping the pixel spacing or usingadditional corrective optics inside the spectrometer.

This partial illumination of each pixel may reduce the effectiveacquisition linewidth of the pixel, which may decrease the fall off Insome embodiments of the present invention where an interleaver is usedfor the POF 108, one half of each pixel in the spectrometer may beilluminated and the fall off may be reduced by a factor of 2,independent of whether one or two spectrometers are used. If bothspectrometers are used, the sampling interval is effectively reduce byhalf, which may double the total imaging depth into the sample as well.

The embodiments illustrated in FIG. 2 similarly use a POF 108 but,instead of two spectrometers 109, 110, a single spectrometer 111 and anoptical switching device 120 are used. This may have the same advantagesof the configuration shown in FIG. 1 in that the fall off may be reducedby a factor of 2 and the depth may be increased by a factor of 2, but doso while only using one spectrometer. In typical implementations, thespectrometer 111 may represent a significant portion of the cost of thesystem and optical switching devices 120 may be cheaper. The opticalswitching device 120 may be selected so that it switches fast enoughthat the spectrometer 111 can still measure twice as many spectra in agiven time and so that the optical switching device 120 has sufficientextinction ratio that light leaking through from the opposite input isnot a significant noise source for the spectrometer 111.

The embodiments illustrated in FIG. 3 include aspects from bothembodiments previously described with reference to FIGS. 1 and 2. In theembodiments of FIG. 3 multiple POFs 130, 131 and 132 are used inconjunction with a combination of spectrometer(s) 135 and 136 and may,optionally, also include one or more optical switching devices 133 and134. At least one spectrometer is used in the embodiments of FIG. 3, butthe number of optical switching devices may range from none to aplurality. The cascaded POFs 130, 131 and 132 split the spectrum intomultiple combs, where ones of the combs are periodic and offset from theother combs. In some embodiments, the POF 130 has a FSR that is half theFSR of POFs 131 and 132 and all the POFs having a finesse of 2. Suchembodiments may split the incoming spectrum into four spectra, with thespacing for each spectra equal to the FSR of POFs 131 and 132 and peakwidths determined by POFs 131 and 132. The ordering of the POFs may varyin some embodiments with a filter with each free spectral range presentin each path.

Once the wavelengths are split into multiple combs, they pass tospectrometers 135 and 136 or through optical switching devices 133 and134 to the spectrometers 135 and 136. In some embodiments, thespectrometers 135 and 136 have a pixel spacing equal to the largest freespectral range of the POFs. Note that the optical switching device 133and 134 may have more than two inputs and one output, for example, anM×N optical switching device could be used to connect M outputs of POFsto N spectrometers. In other embodiments, the optical switching devicesare not included and the outputs of the POFs 130, 131, 132 are connecteddirectly to the spectrometer(s) 135, 136.

All of the outputs of the POF(s) may not be used and the image fall offmay be determined by the filter width of the narrowest POF and the imagedepth may be set by the effective sampling interval, which depends onthe number of spectrometers. In some embodiments, an upgradable systemis provided where one or more POFs and one or more spectrometers and oneor more optical switching devices are installed on day one and, as userrequirements change, additional POFs, optical switching devices and/orspectrometers may be added in whatever configuration is selected for aparticular application.

The embodiments of FIG. 4 are similar to those described with referenceto FIG. 1 with the addition of a tunable POF 137. A tunable POF is onewhere the wavelength comb sectra on a particular output can be shiftedin frequency by some control method. Multiple spectrometers could beused, but some embodiments only include one spectrometer 109 as shown inFIG. 4. The POF 137 may be an interleaver, a Fabry Perot cavity or otherPOF and may be tunable by various means including mechanical,electrical, or optical. In addition, to shifting the spectra, the freespectral range and the finesse of the cavity may be shifted in someembodiments.

In some embodiments the POF 137 is configured to tune fast enough topermit the spectrometer 109 to acquire spectra at a rate sufficient tosupport the desired image rate for the optical engine. In someembodiments, the tunable POF 137 can obtain the same results as thepreviously described combinations of POFs, optical switches andspectrometer, but may allow additional flexibility. Using a tunable POFgenerally involves a trade-off between the number of spectrometers andthe time to acquire an image. In other words, the same result may beachieved with one spectrometer as with multiple spectrometers, but alonger time may be required to collect the full spectra. The embodimentsof FIG. 4 may be easier to scale to higher POF finesse as only onespectrometer and no optical switches may be used. Real time control ofthe image acquisition rate versus image resolution tradeoff may also beprovided.

By varying (increasing) the number of steps in the POF scan, theresolution can be increased and the fall off reduced. If a fast scantime is desired, the number of steps in the POF scan can be reduced.This approach can also be used in conjunction with the architecturesdescribed for the embodiments of FIGS. 1-10, to provide systems whereone or more POFs are tunable and the number of spectrometers can bevaried from 1 to many and the number of optical switches can be variedfrom 0 to many.

Referring now to the embodiments of FIG. 5A, a POF 142 is positioned inthe source arm 140, 141 from the low-coherence source 100. This locationmay provide advantages similar to those described with reference to theembodiments of FIG. 4 but, as the POF 142 is prior to the splitter 101,the power incident on the reference arm 104 and sample arm 105 may bereduced. In the embodiments of FIG. 4, all wavelengths may be incidenton the reference 106 and sample 107 at all times, even though only afraction of the wavelength may pass through the POF 137 and enter thespectrometer 109 at any given time. In cases where there is a limitationon how much power should be incident on the sample 107, such as on thehuman eye for retinal scanning, power that does not pass through the POF137 to the spectrometer 109 is wasted. For the embodiments of FIG. 5A,all power returning from the sample 107 through the splitter 101 andonto the spectrometer path (detector arm) 103 may enter the spectrometer143. It will further be understood that the POF 142 of FIG. 5 may be atunable or fixed POF.

A variant of the embodiments of FIG. 5A is illustrated in FIG. 5B, wherea comb source 145 takes the place of the low-coherence source 100 withthe periodic optical filter 142 shown in FIG. 5A. The embodiments ofFIG. 5B may provide substantially the same advantageous as theembodiments of FIG. 5A while using a different set of sources. The combsources 145 provides a set of wavelengths 1 through N, where there is aspacing between each set of wavelengths and a linewidth associated witheach wavelength. In some embodiments, the spacing between thewavelengths may be constant in wavelength, constant in wavenumber orfrequency, chirped in wavelength or wavenumber, and/or may have someother spacing. There are a variety of ways to build comb sources,including, but not limited to, multi-mode or multi-line lasers,broadband sources with internal filtering and/or feedback, and/or othernonlinear optical devices.

Referring now to the embodiments of FIG. 6, the filtering and tunabilitydescribed previously may be moved inside a spectrometer 150. This mayprovide the same functionality as, for example, the embodiments of FIG.3, but may do so while reducing the number of devices in and the cost ofthe OCT imaging system. The spectrometer 150 may include an input source154, some input or collimating optics 155, a dispersive device 156 and aphotodiode array 151.

As shown in the embodiments of FIG. 6, filtering is added to thespectrometer 150 in the form of a spatial mask 152, which restricts thelight 153 falling on the pixels of the photodiode array 151. The mask152 can be configured so that it is periodic in wavelength and/orchirped so it is periodic in frequency. The size of the holes in thespatial mask 152 can be specified to control the percentage of eachpixel that is lit. This may be used to control the rate of fall off ofthe image.

Furthermore, the spectrometer of the embodiments of FIG. 6 can also beimplemented as a tunable spectrometer, for example, by moving thecollimating optics 155, the diffractive element 156, the spatial mask152, and/or the photodiode 151. Other tunable embodiments may beprovided including adding additional optics, such as a movable mirrorsor prisms, and/or varying the effective index of refraction in thespectrometer 150 by changing the temperature and/or pressure of the gasinside the spectrometer.

Further embodiments are illustrated in the schematic block diagram ofFIG. 7. As shown in the embodiments of FIG. 7, at least two POFs 160 and165 are used in different arms 103 and 104 coupled to the splitter 101.At least one of the POFs 165 is in either the reference arm 104 (asshown in FIG. 7) or the sample arm 105 of the interferometer (opticalengine) and the second POF 160 may be in any of the other three arms102, 103 and 105 that is not occupied by the POF 165. By offsetting thetwo POF's 160 and 165 relative to each other, the effective bandwidth ofthe comb can be reduced below the bandwidth of either POF 160 and 165.As an example, two interleavers (POFs) with the same FSR and finesseesof 2 could be offset by one quarter of the FSR to achieve an effectivefilter width that is one half the width of either interleaver. This mayresult where interference at the spectrometer 161 only occurs forwavelengths that are present in both the reference arm 104 and thesample arm 105 of the interferometer.

By combining the configuration of FIG. 7 with the use of tunable POFs, adevice may be provided where the effective finesse is readilycontrolled. While the embodiments of FIG. 9 discussed below generallyaddress this feature, it may be more difficult to build a POF where thefinesse is adjustable than to build a POF where the output comb istunable in wavelength. By tuning the output comb of both POFs 165 and160 in the embodiments of FIG. 7, the location and width in wavelengthof the light on each photodiode of the spectrometer 161 may becontrolled. This may permit control of the rate of fall off of the imageand the image depth. The tradeoff may be in the amount of power on thephotodiodes in the spectrometer 161 and the time required to build upsufficient data for an image.

Also schematically shown in the embodiments of FIG. 7 are optionaladditional devices 162 and 164 that may be connected to additional portsof the POFs 160, 165, respectively. These additional ports may not beconnected to anything but information is available in the light on theseadditional ports. As such, for example, an additional spectrometer 162could be coupled to the POF 160 and a power or other spectrum monitordevice 164 could be coupled to the POF 165.

Also shown in FIG. 7 are a dotted line graph and a solid line graph. Thesolid line represents a passband shape for the first POF 165 and thedotted line represents the passband shape for the second POF 160according to some embodiments. The combined passband (or the overlap)may give narrower peaks then either POF by itself. Such narrower linesmay be desireable, however, the cost and complexity of a POF generallyincreases with the narrowness of the lines. Thus, a two POF design mayprovide narrower lines at a lower cost. Also if one or both POFs aretunable, then in some embodiments the overlap can be adjusted in amanner determined based on the particular application of the opticalengine.

Further embodiments are illustrated in the schematic block diagram ofFIG. 8. In the embodiments of FIG. 8, only one pass of each POF 171 and177 is used. As shown in FIG. 8, the POFs 171 and 177 are used inconjunction with circulators 172 and 175 so that light from thelow-coherence source 100 through the first coupler splitter 101 passesthrough the POFs 171 and 177 and light returning from the reference 106and the sample 107 does not pass through POFs 171 and 177 again on thereturn, but is diverted by the circulators 172 and 175 to the secondcoupler splitter 180 on optical connections 176 and 179 and, hence, tothe spectrometer 170. This may be desirable if the POFs 171 and 177 havehigh insertion loss, particularly in comparison to the circulators 172and 175. Various aspects of tenability over a range of wavelengthspresent in the embodiments of FIG. 7 may also be used in the embodimentsof FIG. 8.

Also schematically shown in the embodiments of FIG. 8 are optionaladditional devices 173 and 178 that may be connected to additional portsof the POFs 171 and 177, respectively. These additional ports may not beconnected to anything, but information is available in the light onthese additional ports.

The embodiments of FIG. 9 include one or more optical multiplexers anddemultiplexers as the optical filters. Multiple sources 200, 201 and 202are multiplexed by the optical multiplexer 206 onto a single fiber inthe source arm 102 of the optical engine (interferometer) to provide asuper-source and then demultiplexed by the optical demultiplexer 207 inthe detector arm 103 onto multiple detectors 203, 204 and 205. Themultiplexer 206 could be an arrayed waveguide (AWG), concatenated thinfilm filters, fiber Bragg gratings, power couplers, and/or the like. Thedemultiplexer 207, in some embodiments, has some wavelengthdiscrimination capability rather than using a simple splitter.

The sources may be tunable lasers with detectors (as shown in FIG. 9)and/or super luminescent diode sources combined with spectrometers onthe detection side. The number of sources (either swept or broadband)and detectors (individual or in spectrometers) does not need to matchalthough, in some embodiments, any detector, whether individual, in anarray and/or part of a spectrometer, sees light only from either onesource or only at one wavelength.

As noted previously, the effective wavelength spread seen by a detectormay determine the rate of image falloff. The effective wavelength spreadmay be set by the source (narrowband swept source laser) or by filteringin the system. With this approach, any of the previous POFconfigurations may also be used. POFs may be added in the source arm102, the reference arm 104, the sample arm 105, the detector arm 103, orany combination thereof. Any and all of the POFs could be tunable aswell as the sources given by 200, 201 and 202 and the detectors 203, 204and 205.

As illustrated in FIG. 10, further embodiments are similar to thedescription of the embodiments of FIG. 9 above, but with the use of aset of circulators 218, 219 and 220 and an opticalmultiplexer/demultiplexer 216 instead of the optical multiplexer anddemultiplexer 206 and 207. Each circulator 218, 219 and 220 is coupledto an associated source 210, 212 and 214 and an associated detector 211,213 and 215. This difference may provide flexibility in design selectionbased on the cost of circulators versus optical multiplexers ordemultiplexers and the potential requirements that the multiplexer anddemultiplexer be identical.

Also schematically shown in the embodiments of FIG. 10 is an optionaladditional device 217 that may be connected to an additional port of thesplitter/coupler 101. This additional port may not be connected toanything but information is available in the light on this additionalport.

Further embodiments are shown in FIG. 11, which embodiments are similarto the embodiments of FIG. 5A, but a swept laser source 300 is used withone or more photodiodes 320 instead of using a broadband or lowcoherence source 100 with a spectrometer 143 as shown in FIG. 5A. Theembodiments of FIG. 11 may provide substantially the same advantages asthe embodiments of FIG. 5A, such as controlling the effective linewidthand minimizing optical power on the sample while providing a new set ofsources that can be used in the system implementation. By using the POF310, the linewidth of the laser can be broadened to approximately thefree spectral range (spacing) (FSR) of the POF 310 while the systemstill may have the effective linewidth characteristics of the linewidthof the POF 310.

The embodiments of FIG. 12 are similar to the embodiments of FIG. 11.However, FIG. 12 uses a swept laser source 300, but with a POF 311 inthe detector arm 103 rather than in the source arm 141. The POF 311 maybe connected to one or more photodiodes 320, 321. The embodiments ofFIG. 12 may have substantially the same advantageous as the embodimentsof FIG. 11 in that the effective linewidth of the source can becontrolled while also offering the ability to increase the imaging depthby acquiring light from more than one port of the POF 311.

FIG. 13 illustrates further embodiments where multiple SLDs 400, 401 aremultiplexed together by a multiplexer 420 to create a source which mayhave enhanced characteristics, for example, a broader total bandwidth.Similarly, one or more spectrometers 410, 411 may be used in thedetector arm connected together by a demultiplexer 421. In theembodiments of FIG. 13, the multiplexer 420 and demultiplexer 421 maycombine and separate the light from various ports based on wavelengthand/or may simply be couplers that mix the light together based on powerand/or other devices for combining and separating light. The multiplexer420 and the demultiplexer 421 may also have properties of a POF where aneffective linewidth is modified for one or more of the detector(s) inthe spectrometer(s) 410, 411. In some embodiments, there may be one ormore separate POF(s) in the source arm 102 and/or the reference arm 104and/or the sample arm 105 and/or the detector arm 103.

Yet further embodiments of the present invention are illustrated in FIG.14, where one or more source subsystems 500,501 are connected to asource arm 102, which may or may not include optical multiplexer(s) 520and POF(s) 530. The reference arm 104 and the sample arm 105 may or maynot include POF(s) 532 and 533. The detector arm 103 is illustrated asincluding one or more detector subsystem(s) 510, 511 and may or may notalso include optical demultiplexer(s) 521 and/or POF(s) 531. The sourcesubsystems 500, 501 generate light and may include SLD's, swept lasersources, comb sources, other light sources, and/or a mixture of variouslight sources. Likewise the detector subsystems 510, 511 detect lightand may include single photodiodes, photodiode(s) with filters,spectrometers, other light detection devices and/or mixtures of variousdetector implementations. In some embodiments, a system designrequirement is that there are acquisitions from one or more detectorsthat can be mapped relative to each other in wavelength (or wavenumber).This mapping may be some combination of time and wavelength, such asreading a line of data from a spectrometer or a time series of data froma detector, but can be any combination or implementation such that thereis a defined mapping and the system can sample light from knowwavelengths or wavelength spacings at know times or time spacings.

In some embodiments of the present invention, optical engines (systems)include an optical source, a plurality of optical detectors, a pluralityof interferometers and two or more optical filters. One or more of theoptical filters may be POFs that are periodic in frequency. POF(s) maybe in the spectrometer arm and two or more spectrometers may beincluded. POF(s) may be in the spectrometer arm and one or more opticalswitching element(s) and one or more spectrometers may be included.POF(s) may be in the source arm. One or more of the POFs may be tunable.

In some embodiments of the present invention, one or more POFs are inthe reference arm and one or more POFs are in the sample arm andcirculators are between the POFs and reference or sample. In suchembodiments, light only traverses the POF once and goes to thespectrometer via the circulator and another combiner. The spectrometermay be a tunable spectrometer.

In some embodiments, the POF is provided as a sub-pixel mask that may beused in combination with a tunable spectrometer. The spectrometer mayhave chirped pixels. The pixels may be chirped by having an increasingwidth. The pixels may be chirped by having a constant width and anincreasing gap. Two or more POFs may be included that may be offset infrequency. One or more POF(s) may be in the reference arm and one ormore POF(s) may be in the sample arm. One or more of the POFs may betunable.

In some embodiments, one POF is in the sample arm and one POF is in thespectrometer arm. One POF may be in the reference arm and one POF may bein spectrometer arm. One or more of the POFs may be tunable.

In some embodiments of the present invention, optical engines (systems)include two or more optical sources, a plurality of muxing devices forthe sources, a plurality of optical detectors, a plurality ofinterferometers, and a plurality of optical filters. The sources may belasers and one or more of the lasers may be tunable. The muxing devicesmay be configured to provide power muxing, polarization muxing and/orwavelength dependent muxing.

In some embodiments, filters are in the detector arm. The filters mayinclude arrayed waveguides (AWGs), thin film filters, echelon gratingsand/or Fiber Bragg Gratings. The sources may be superluminescent diodes(SLDs) that may be tunable.

It will be understood that, with a FD-OCT system, the detector systemgenerally knows what wavelength spacings it is interrogating so that itcan generate a set of intensity values as a function of wavelength. Thisinformation may then be fed into an FFT to generate the depth image. Assuch, the fall-off in such imaging systems is generally determined bythe effective linewidth (bandwidth) of the light seen by the detectorsystem when an amplitude measurement is taken. Previous approaches toobtain a desired effective linewidth included using a swept source wherethe linewidth of the source becomes the effective linewidth and using aspectrometer where the linewidth viewed by a single detector in thearray becomes the effective linewidth. As described above, variousembodiments of the present invention provide a variety of differentapproaches to set the effective linewidth. Some embodiments of thepresent invention may provide a narrower (less fall-off) effectivelinewidth than the previously known approaches.

In addition, in some embodiments, there may also be other advantages inusing combinations of sources and detectors to create a source subsystemand a detector subsystem that may have better performance, including:(1) higher output power, (2) faster sweep rates, (3) larger totalbandwidths, and/or (4) cheaper construction costs. As such, someembodiments of the present invention may provide better performancerelative to the issue of fall-off while allowing commercial building ofsuch FD-OCT systems. As described above some embodiments involve the useof optical filters and/or combinations of sources and/or detectors toprovide imaging systems that may include a source subsystem, a detectorsubsystem, a reference arm, and a sample arm connected to aninterferometer where the design of the source subsystem and the detectorsubsystem produce a narrower effective linewidth than previously knownapproaches.

In some embodiments, a source selection rather than optical transmissionpath design may provide such effective linewidth narrowing, for example,through the use of a comb source. A comb source with a spectrometer mayprovide a solid state system with no sweeping while, nonetheless,providing a better falloff as the linewidth of the comb source may beused to set the fall-off instead of or in addition to the linewidth of apixel in the spectrometer.

In the drawings and specification, there have been disclosed typicalillustrative embodiments of the invention and, although specific termsare employed, they are used in a generic and descriptive sense only andnot for purposes of limitation, the scope of the invention being setforth in the following claims.

That which is claimed is:
 1. A frequency domain optical coherencetomography imaging system, the system comprising: an optical splitter; areference arm and a sample arm coupled to the optical splitter; a sourcearm coupled to the optical splitter and including an optical sourceemitting across a total bandwidth that partially defines an axialresolution of the system, wherein the optical splitter receives lightfrom the source arm and directs a first portion of the light from thesource arm into the reference arm and a second portion of light from thesource arm into the sample arm; an optical coupler that receives lightfrom the reference arm and from the sample arm, the optical couplerbeing one of the same device as the optical splitter or a differentdevice from the optical splitter; a detector arm coupled to the opticalcoupler, the optical coupler directing a portion of light received fromthe reference arm and the sample arm to the detector arm; and at leastone optical detector in the detector arm, the detector arm receivinglight from the optical coupler and directing the light from the opticalcoupler to the at least one optical detector; wherein the at least oneoptical detector detects a plurality of discrete spectral elements fromwithin the total bandwidth of the optical source; wherein aconfiguration of the sample of the plurality of spectral elementscomprises a mapping of a spatial or temporal readout of the at least oneoptical detector to the wavelength or frequency of a correspondingspectral element; wherein at least one of the source arm, the referencearm, the sample arm and the detector arm comprises a periodic opticalfilter having an FSR less than a total bandwidth of the source and afinesse greater than two to provide a plurality of spectrally separatedspectral elements; wherein the at least one optical detector isconfigured to sample a plurality of spectral elements at one ofsubstantially equal intervals to the FSR or intervals greater than theFSR of the filter; wherein a spectral spacing is provided between eachof the sampled plurality of spectral elements and a bandwidth isassociated with each of the sampled plurality of spectral elements;wherein the spectral spacing between each of the sampled plurality ofspectral elements partially defines an image depth of the system and thebandwidth associated with each of the sampled plurality of spectralelements partially defines the image depth of the system; and whereinthe bandwidth of each of the plurality of spectral elements sampled bythe at least one optical detector is less than or equal to one-half thespectral spacing between each of the sampled plurality of spectralelements and less than or equal to one-half the total bandwidth of theoptical source.
 2. The system of claim 1, wherein the detector armcomprises a spectrometer including a plurality of optical detectors. 3.The system of claim 2, wherein the optical source comprises a broadbandsource.
 4. The system of claim 3, wherein the spectrometer comprises adispersive element that disperses the bandwidth of the optical sourceonto a plurality of optical detectors, wherein spatial orientation ofthe plurality of optical detectors is substantially evenly spaced withrespect to a frequency of the dispersed optical source.
 5. The system ofclaim 2, wherein the optical source comprises an optical comb source. 6.The system of claim 5, wherein the optical comb source comprises aperiodic optical filter.
 7. The system of claim 6, wherein thespectrometer comprises a dispersive element that disperses the bandwidthof the optical source onto a plurality of optical detectors, wherein aspatial orientation of the plurality of optical detectors issubstantially evenly spaced with respect to the frequency of thedispersed optical source.
 8. The system of claim 2, wherein thespectrometer comprises a spatial mask proximate the optical detectors.9. The system of claim 1, wherein the optical source comprises a sweptsource.
 10. The system of claim 9, wherein the swept source comprises aswept comb source.
 11. The system of claim 10, wherein the swept combsource comprises a swept source and a periodic optical filter.
 12. Afrequency domain optical coherence tomography imaging system, the systemcomprising: an optical splitter; a reference arm and a sample armcoupled to the optical splitter; a source arm coupled to the opticalsplitter and including an optical source emitting across a totalbandwidth that partially defines an axial resolution of the system,wherein the optical splitter receives light from the source arm anddirects a first portion of the light from the source arm into thereference arm and a second portion of light from the source arm into thesample arm; an optical coupler that receives light from the referencearm and from the sample arm, the optical coupler being one of the samedevice as the optical splitter or a different device from the opticalsplitter; a detector arm coupled to the optical coupler, the opticalcoupler directing a portion of light received from the reference arm andthe sample arm to the detector arm; at least one optical detector in thedetector arm, the detector arm receiving light from the optical couplerand directing the light from the optical coupler to the at least oneoptical detector; wherein the at least one optical detector detects aplurality of discrete spectral elements from within the total bandwidthof the optical source; wherein a configuration of the sample of theplurality of spectral elements comprises a mapping of a spatial ortemporal readout of the at least one optical detector to the wavelengthor frequency of a corresponding spectral element; wherein at least oneof the source arm, the reference arm, the sample arm and the detectorarm comprises means for providing an FSR less than a total bandwidth ofthe source and a finesse greater than two to provide a plurality ofspectrally separated spectral elements; and means for sampling aplurality of spectral elements at one of substantially equal intervalsto the FSR or intervals greater than the FSR of the filter; wherein aspectral spacing is provided between each of the sampled plurality ofspectral elements and a bandwidth is associated with each of the sampledplurality of spectral elements; wherein the spectral spacing betweeneach of the sampled plurality of spectral elements partially defines animage depth of the system and the bandwidth associated with each of thesampled plurality of spectral elements partially defines the image depthof the system; and wherein the bandwidth of each of the plurality ofspectral elements sampled by the at least one optical detector is lessthan or equal to one-half the spectral spacing between each of thesampled plurality of spectral elements and less than or equal toone-half the total bandwidth of the optical source.
 13. The system ofclaim 12, wherein the means for providing comprises a periodic opticalfilter having an FSR less than a total bandwidth of the source and afinesse greater than two to provide a plurality of spectrally separatedspectral elements.
 14. The system of claim 12, wherein the means forsampling comprises the at least one optical detector configured tosample a plurality of spectral elements at one of substantially equalintervals to the FSR or intervals greater than the FSR of the filter.